The present invention pertains generally to laser beam machines and more particularly to their optical fiber wiring and beam output optical assemblies having improved structures.
It is known that a carbon dioxide gas (CO.sub.2) laser, which features high beam quality (i.e., light-condensing capability), is an excellent light source for use in laser beam cutting of metal plates and sheets. Various types of CO.sub.2 laser beam machines using mirrors and infrared-transparent optical fibers have thus far been proposed and developed for practical applications. On the other hand, yttrium-aluminum-garnet (YAG) lasers have been used in marking and trimming applications, for instance, because they have been able to provide good beam quality only in low-power ranges. However, recent technological advances have made it possible to develop YAG lasers which would offer as good beam quality as the CO.sub.2 lasers even at output power levels exceeding 100 W. As a result, it has become possible for the YAG lasers, which utilize visible-light-transparent optical fibers featuring a high degree of operability and multi-purpose applicability, to achieve such cutting performance that is comparable to the CO.sub.2 lasers.
FIG. 9 is a perspective diagram illustrating a light-condensing head positioning mechanism of a CO.sub.2 laser beam machine utilizing an infrared-transparent optical fiber, of which example is disclosed in Japanese Patent Application No. 57-124586. In FIG. 9, there are provided a flexible optical fiber cable 1 containing an infrared-transparent optical fiber measuring 10.6 .mu.m in wavelength at CO.sub.2 laser connected to a laser oscillator (not shown) and a beam output optical assembly 2 connected to the end of the optical fiber cable 1. Designated by the numeral 3 is a table on which a workpiece W, e.g., a piece of sheet metal, is fixedly mounted. Designated by the numeral 4 is a positioning mechanism for moving the beam output optical assembly 2 above the workpiece W in a two-dimensional processing pattern. The positioning mechanism 4 includes a first carriage member 4-2 which moves along a pair of rails 4-1 and a second carriage member 4-3 which moves at right angles to the rails 4-1 along another pair of rails 4-4. The beam output optical assembly 2 is mounted to the second carriage member 4-3.
Described in the following is how the CO.sub.2 laser beam machine of the above construction operates when marking a graphic or text pattern on the workpiece W. A marking process is commenced by first positioning the beam output optical assembly 2 just above a specified location of the workpiece W. A controller (not shown) transmits a control signal produced in accordance with readily entered marking pattern data. This control signal controls opening and closing operations of a shutter provided inside a CO.sub.2 laser oscillator (not shown) in order to intermittently deliver and interrupt laser light. The laser light of an appropriate energy level transmitted to the beam output optical assembly 2 via the optical fiber cable 1 is switched on and off in this manner. The beam output optical assembly 2 is moved by the first carriage member 4-2 and second carriage member 4-3 of the positioning mechanism 4 above the workpiece W. When exposed to a laser beam radiated from the beam output optical assembly 2, resulting heat causes an exposed surface zone of the workpiece W to evaporate or melt so that the specified pattern is marked on the workpiece W in accordance with the movement of the beam output optical assembly 2 and the laser light switching on/off sequence.
The above-described CO.sub.2 laser beam machine employs an optical fiber for laser light transmission. Unlike laser machining systems using mirrors for laser beam transmission, this type of laser beam machine can not be used for cutting a steel plate, for instance, due to a large transmission loss of the 10.6 .mu.m infrared-transparent optical fiber. Development of a low transmission loss optical fiber has therefore been awaited. YAG laser machining systems also have a similar problem. Although transmission loss of optical fibers used for carrying visible light is sufficiently low, output power of a YAG laser that can provide satisfactory beam quality is limited to a few tens of watts.
Generally, it is necessary to eject an assist gas together with a laser beam through a coaxial nozzle to realize a high-quality cutting process. Different assist gases are used for different types of workpiece. As an example, oxygen is most often used for steel plates and stainless steel plates. In this example, it is possible to increase the cutting speed and reduce the amount of dross (i.e., waste product deposits caused in cutting operations) by increasing the gas pressure. Also effective for making efficient use of laser beam energy, increasing the cutting speed and reducing thermal distortion is to converge the laser beam on as small a spot as possible. It is a common practice in laser beam machining to construct beam output optical systems in consideration of the above aspects.
FIG. 10 is a cross-sectional view showing the structure of a conventional beam output optical assembly 2 used in YAG laser beam machines which employ optical fibers transparent to visible light. In FIG. 10, designated by the numeral 1 is an optical fiber cable containing an visible-light-transparent optical fiber in a flexible protective jacket. The beam output optical assembly 2 comprises a nozzle centering mechanism 12, a beam output optical assembly housing 2-1 provided with an assist gas inlet 15, a collimator lens 14 and a condenser lens 13. Designated by the numeral 11 is an optical fiber joint which connects the optical fiber cable 1 to the beam output optical assembly 2. The optical fiber joint 11 comprises an optical connector 11-1 including a plug 11-1a and a receptacle 11-1b. The receptacle 11-1b is screwed to one end of the beam output optical assembly housing 2-1. Mounted to the beam output optical assembly housing 2-1 at the opposite end to the optical fiber joint 11, the nozzle centering mechanism 12 includes a nozzle 12-1, a first nozzle holder 12-2, nozzle adjusting screws 12-3 mounted on the first nozzle holder 12-2 and a second nozzle holder 12-4.
Operation of the beam output optical assembly 2 of FIG. 10 is now described. The laser light transmitted through the optical fiber cable 1 is emitted from the end of the optical fiber at the optical connector 11-1 of the optical fiber joint 11. The laser light propagates through the air in a diverging pattern until it reaches the collimator lens 14, which brings the laser light into a parallel beam. The condenser lens 13 causes the parallel beam to converge so that the laser beam outgoing through the nozzle 12-1 is focused on the surface of a workpiece W. Before starting a cutting operation, a machine operator carries out an adjustment for aligning the laser beam with the center of the nozzle 12-1 by turning the nozzle adjusting screws 12-3. With this adjustment, an assist gas injected from the assist gas inlet 15 is uniformly blown onto a currently processed spot to accomplish a successful cutting operation.
The conventional laser beam machines constructed as described above provide limited machining areas using low-power laser beams. Their optical fiber cables are therefore simple in construction and suspension-type cable arrangements are sufficient to support them. With the advent of YAG lasers featuring high beam quality, attempts have been made to use them for high-speed cutting operations like CO.sub.2 lasers. If a YAG laser is used for high-speed cutting, however, optical fibers connected to its beam output optical assembly may break due to severe swinging and vibration, resulting in laser light leakage into surrounding environments. Generally, the broader the machining area of a laser beam machine which produces high-power laser light applicable to metal cutting operations, the higher the risk of cable breakage. Other problems of prior art techniques are that it is difficult to trace down optical fiber breakage and maintain an optical fiber cable and that the optical fiber cable can hardly be arranged in a compact manner because its permissible bending radius differs from those of assist gas pipes, power cable and signal lines.
The aforementioned conventional beam output optical assembly 2 has at its foremost end the nozzle centering mechanism 12. This increases the physical dimensions of the beam output optical assembly 2 and limits the machining area. Since the assist gas is injected under about 10 kg/cm.sup.2 the atmospheric pressure at maximum into a space confined by the nozzle 12-1 having a small orifice typically measuring about 2 mm in diameter, a thick single lens is used as the condenser lens 13 to avoid its physical damages. This poses another problem that it is necessary to use an expensive aspherical lens in order to prevent spherical aberration that can lead to degradation of laser light converging performance.